Whisk yolk and 1/4 cup sugar in large bowl until very thick and pale, about 2 minutes. Combine 2 cups cream and milk in heavy large saucepan. Scrape in seeds from vanilla bean; add bean. Bring to boil. Gradually whisk into yolk mixture. Return to pan and stir over medium heat until custard thickens and coats spoon, about 4 minutes (do not boil). Remove from heat. Mix in caramel. Strain into bowl. Chill until cold, stirring often.

Bake macaroons until puffed and dry-looking on top, about 12 minutes. Cool on sheets on racks. Using spatula, transfer macaroons to work surface.

Spoon 1/4 cup ice cream onto flat side of each of 5 macaroons. Top each with flat side of another macaroon; press lightly. Place sandwiches on baking sheets; cover and freeze. (Can be made 1 day ahead.)